1. Field of the Invention
The present invention relates to an adhesive composition and to an adhesive sheet and an adhesive optical component using the adhesive composition. More particularly, the present invention relates to an adhesive composition which exhibits excellent adhesion with adherends and excellent stress relaxation and can follow change in the dimension of substrates and to an adhesive sheet and an adhesive optical component such as a polarizing plate which comprise the adhesive composition.
2. Description of Related Art
When a sheet of an organic material is stuck to an adherend having a hard surface such as glass, ceramics and metals via an adhesive, undesirable phenomena such as peeling and lifting of end portions of the sheet from the adherend tend to take place with passage of the time. To overcome the problem, in general, an adhesive having great adhesive ability enhanced by increasing the molecular weight or the degree of crosslinking of a resin component constituting the adhesive is used. However, when contraction or expansion of a substrate takes place by change in the temperature or the humidity, the adhesive cannot follow the change in the dimension of the substrate since such an adhesive is hard and has small elasticity. Therefore, the use of such an adhesive causes various problems although the holding ability can be enhanced.
Some optical components are used after a polarizing plate is attached to the surface. Typical examples of such optical components include liquid crystal cells in liquid crystal displays (LCD). In general, a liquid crystal cell has a structure in which two transparent electrode substrates having an oriented layer are placed in a manner such that a specific gap is formed between them and the oriented layers face to each other at the inside, peripheral portions are sealed, a liquid crystalline material is placed inside the gap between the transparent electrode substrates and a polarizing plate is disposed on each outer surface of the two transparent electrode substrates via an adhesive layer.
FIG. 1 shows a perspective view exhibiting the structure of an example of the polarizing plate described above. As shown in FIG. 1, the polarizing plate 10 has a substrate having a three-layer structure in which triacetylcellulose (TAC) film I 2 and TAC film II 2′ are laminated on the faces of a polarizing plate 1 made of polyvinyl alcohol. On one face of the substrate, an adhesive layer 3 for adhering the substrate with an optical component such as a liquid crystal cell is formed. A release sheet 4 is attached to the adhesive layer 3. In general, a film for protecting the surface 5 is disposed on the face of the polarizing plate opposite to the face having the adhesive layer 3.
When the above polarizing plate is attached to the above liquid crystal cell, the release sheet 4 is removed first, then the polarizing plate is attached to the liquid crystal cell via the exposed adhesive layer and the film for protecting the surface 5 is removed.
The polarizing plate which is attached to the liquid crystal cell via the adhesive layer has the three-layer structure described above. Due to the properties of the materials, the polarizing plate has poor dimensional stability and, in particular, change in the dimension by contraction or expansion is great in the environment of a high temperature or a high temperature and a high humidity.
However, since, in general, an adhesive having a great adhesive ability is used in the above polarizing plate as described above, stress caused by the change in the dimension of the polarizing plate cannot be absorbed and relaxed by the adhesive layer although lifting and peeling caused by the change in the dimension of the polarizing plate can be suppressed. More specifically, TAC film II 2′ at the front face tends to contract or expand due to change in the humidity and the temperature. On the other hand, TAC film I 2 cannot contract or expand easily since TAC film I is firmly adhered to the liquid crystal cell via the adhesive layer 3 and the adhesive layer cannot flexibly follow the change in the dimension. As the result, ray passing through TAC film I toward TAC film II cannot proceed straight. This causes undesirable phenomena such as formation of blank spots.
To overcome the above problem, heretofore, a plasticizer is added to the adhesive so that the adhesive is flexible to a suitable degree and stress relaxation takes place. However, the adhesive containing a plasticizer has drawbacks in that the plasticizer bleeds out and that the adherend is stained with the plasticizer when the polarizing plate is peeled by the bleeding out. As another method to overcome the above problem, the number of crosslinking in the adhesive is decreased by using a polyfunctional crosslinking agent having a functionality of three or greater. However, the holding ability, i.e., the adhesion with the adherend, inevitably decreases in this case and problems such as lifting and peeling of the polarizing plate tend to arise with passage of the time.